1. Field of the Invention
The present invention relates generally to well logging tools for the acquisition of data in an underground borehole. The present invention relates more specifically to a tool, and a method for using the tool, to determine the dip angle of underground formations traversed by a cased or uncased borehole.
2. Description of the Related Art
The oil and gas industry has devoted much effort to the development of various devices and methods for identifying, recording and analyzing the characteristics of underground formations traversed by boreholes. This information on the characteristics of underground formations is critical to a real time determination of the prospects for locating oil or gas in a drilled location. Extensive studies of geological formations have allowed oil and gas operators to improve the chances of locating and reaching oil and gas bearing formations simply by maintaining accurate information on the composition and structure of the various formations traversed and followed in the process of drilling the borehole.
There are many different well logging techniques and tools available on the market. These tools and techniques generally fall into three or four different categories of investigation. Techniques such as the detection and measurement of resistivity, induction, conductivity, acoustic and electromagnetic field changes to radiation measurements, both active and passive, are utilized in typical well logging systems. These methods seek to identify the nature or composition of the formations surrounding the borehole at any given point, to identify the distance into the borehole that the tool is positioned at, to identify the angle of inclination for the borehole itself, to identify the directional orientation (typically from a reference azimuth or from a magnetic north) of the borehole.
Also of great importance to many well logging techniques, and the primary subject of the present invention, is a determination of the angle at which the underground formations are oriented at the point at which the borehole traverses the formation. This so-called “dip angle” allows the oil and gas operators to not only characterize the nature of the formation surrounding the borehole but additionally to identify the likely direction that the surrounding formations follow or the likely location of other formations of particular interest that are known to be adjacent the referenced formation. Such information has become an aid in determining structure and is particularly relevant to drilling techniques in so called “directional drilling” in angled and horizontal wells where geosteering methods are implemented. This is typically the case in multiwell offshore platforms that necessarily run directional boreholes in order to accommodate a number of wells from a single platform. Rather than working with strictly vertical wells, many current oil and gas explorations incorporate steerable well drilling techniques that allow the oil and gas operators to respond to formation structures and dip angles in a manner that permits a change in the direction of the well drilling, often during the drilling operation.
A “dip log” is the recording of information from which the angle and direction of geological bedding planes may be determined. A dip log device is typically used within an open well borehole without casing primarily because existing techniques rely upon direct contact between the sensors measuring the formation and the borehole wall/surface of the formation. Analysis of the information provided by a dip log device makes possible the identification of reefs, channels and faulting, depth formations around salt domes, and other structural anomalies critical in the analysis of oil field geology.
The standard dip log device has either three or four arms positioned radially about a central cylinder and that extend out to make contact with the borehole wall surface. Attached to each arm is a small flexible rubber pad designed to give good contact with the wall of the borehole. Molded into the face of each pad are several small electrodes that operate in pairs to establish a flow of electrical current in the formation upon contact with the electrodes. The arms track the sides of the borehole as the logging tool moves up or down so that the conductivity or resistivity in each area can be measured. Not only are distinctions made in the conductivity or resistivity according to depth but more specific distinctions are drawn between measurements made by one set of electrodes and those made by a second (or third or fourth) set of electrodes positioned radially distinct from the first. In this manner not only can the depth and “thickness” of the formation be determined but some localized inclination can be measured as well. It is this inclination that can be most useful in geosteering operations and in characterizing an approach to a pay zone formation.
The upper section of a dip log device will typically contain a physical orientation mechanism that continuously establishes the position and directional orientation of the instrument with respect to both the gravitational reference and an azimuthal reference (magnetic north). The dip log is normally run after the other open hole logs have been completed. The instrument is attached to the end of a logging cable (wire line) and is suspended and lowered into the borehole with arms in a closed position (see FIG. 1). Once the device is at the bottom of a hole or at the point where the deepest interval is to be logged, a calibration of the measuring circuits is made. The arms are then extended placing the face of each measuring pad in direct contact with the borehole wall. (The new device of the present invention does not require surface contact with the formation as do all existing devices. Tool centralizers in the present invention keep the tool in a proper position.) A survey is then made of the variations and changes in the resistivity or conductivity characteristics of the surrounding traversed formations as the assembly is drawn upwards through the borehole.
By tracking both the radial orientation and the conductivity trace of each the four sensor bearing arms (or three as the case may be), detailed information about the compositions and inclinations of the formational structures that intersect the borehole can be determined. In general, however, this information is readily available only by utilizing a wire line well logging system, as the types of sensors required are typically too fragile to withstand the measurement while drilling environment.
Radiation based well logging, as an alternative to contact resistivity based systems, generally involves either the measurement of natural radiation from the geologic structures intersecting the borehole, or a responsive measurement made after a radioactive source is lowered into the borehole. In either case, measurements of radiation are made and an interpretation of the various levels of the different types of radiation is used to indicate the nature of the structures immediately adjacent to the well logging device. A gamma ray log is considered primarily a shale-locating log while neutron logs provide information on the lithology and the porosity of the formation, thereby characterizing the hydrogen richness of the adjacent formations.
Radiation logs are generally nondirectional in the sense that no specific orientation within the borehole is determined and only a depth of the formation measurement is associated with the radiation measurement.
Resistivity and conductivity logging, as discussed above, have serious limitations. Resistivity and conductivity logging are, for example, ineffective in metal cased boreholes. Consequently owners of older, depleted wells lack dependable logs run prior to completion and often lack other adequate drilling records. These older wells are therefore at a loss to determine at what depth they could tap potential oil producing strata before abandoning their wells. Radioactive well logging, in part, is intended to address this problem.
The equipment necessary for a radiation based well log is relatively straightforward. As with dip log methods described above, logging is completed during the upward trip of a wire line tool in order to maintain tautness and to insure that the entire length of cable is under proper tension. Registration of the radioactivity measurements is generally accomplished at the surface in an instrument truck containing the necessary amplification and recording equipment. Signals within the tool are converted to suitable electrical signal form and are either recorded or transmitted up the wire line for analysis. The signal analysis process identifies deflections along the horizontal axis of a chart that measures the intensity against the depth (into the ground) within the borehole. Two or more radioactivity logging curves may be recorded simultaneously in this manner.
All applicable radiation detectors produce signals that are unsuitable for direct transmission over the hoisting cable, the subsurface equipment necessarily contains some amplifying or signal conversion devices. This in turn requires electrical power for operation that must be supplied through the cable from the surface.
As indicated above gamma ray logging is considered primarily a shale-locating log. It should be recognized that there are two types of gamma log methodologies; a first involves passing natural radiation measurements and a second involves an active induced radiation. Shales and certain evaporates normally emit a higher level of natural gamma radiation than do sandstones and carbonates. This fact implies that low gamma ray counting rates are related to nonshales and high gamma ray counts are related to shales. The amplitude of a gamma ray curve therefore identifies such strata. Neutron logs, as mentioned above, provide information on the lithology and the porosity of the formations. Neutron logs respond to the fundamental formation property of “hydrogen richness.” If all the formation's hydrogen is contained in the form of liquids, and if these liquids completely occupy the total pore volume, hydrogen richness is an index to porosity. A high neutron counting rate indicates low absorption, low hydrogen content in the formation and a low porosity value. Similarly, a low neutron-counting rate implies high absorption, high hydrogen content in the formation and a high porosity value. Water and oil are substantially the same in hydrogen content, while gas is considerably lower. Consequently a neutron measurement is often able to distinguish between liquid and gas saturated intervals.
Various attempts have been made in the past for incorporating radiation based well logging into a system for measuring the dip angle of the surrounding formation. U.S. Pat. No. 2,316,361 issued to Piety on Apr. 13, 1943 entitled METHOD AND APPARATUS FOR SURVEYING WELLS describes improvements in radiation based well logging that allows a determination of the depth at which the sensor instrumentation passes from one geological strata to another. Most important in the Piety patent is the ability to determine the angular distribution of the radiation received at the sensor. The invention provides a method and apparatus for determining the “strike” and approximating the dip of a stratum pierced by a single borehole. The “strike” of a formation is defined as the horizontal direction of a formation bed as measured at right angles to the dip of the bed. The method is based on the difference in the radioactivity of the adjacent strata. The Piety invention achieves this function by combining a determination of the depth of the sensor with the ability to distinguish from which angular direction the sensor is measuring the radiation. The device described utilizes a magnetic compass orientation to determine the angular position of the sensor in the hole. Such a magnetic orientation device would not be functional in typical cased boreholes.
U.S. Pat. No. 2,464,930 issued to Herzog on Mar. 22, 1949 entitled METHOD AND APPARATUS FOR DETERMINING THE INCLINATION OF SUBSTRATA describes a system for providing an effective way of measuring the angle of inclination of formation within a borehole that may be either cased or uncased by providing a radiation source that is lowered into the borehole and utilizing a number of radiation detectors that discriminate the angle from which the radiation is received back. In the preferred embodiment of the Herzog design, three detectors that have radiation shields such that they receive and detect radiation coming from a 120 degree sector opposite the detector. The patent also discusses the use of a magnetic compass or a gyroscopic orientation device.
U.S. Pat. No. 2,725,486 issued to Walstrom on Nov. 29, 1955 entitled METHOD AND APPARATUS FOR WELL LOGGING utilizes a number of radioactive sources that track the interior borehole wall much as do the arms of a standard dip log device. The patent discloses in the first section the shortcomings of electrical resistivity based callipering methods that utilize the well known wall engaging arms through mechanical linkages. The patent instead provides a radiation detector isolated from contact with the borehole walls and the fluids contained in it and a number of radioactive sources, each of which is movable by the wall engaging arms. Although an important objective of the Walstrom invention is to provide a means for determining changes in the diameter of the borehole, a second form of the device is identified as being particularly useful in determining the dip of strata transversed by the well bore. The radiation detecting means includes an individual detector for each of the plurality of radioactive sensors.
U.S. Pat. No. 2,967,933 issued to Scherbatskoy on Jan. 10, 1961 entitled DIP DETERMINATION describes a method and apparatus for measuring the inclination or a dip of a formation or stratum that is transversed by a borehole. The device is intended to be utilized in a borehole that is either cased or uncased and functions by providing a source of neutrons with a number of detectors that can radially detect differences in the reaction of the formation to the neutron bombardment. As indicated in Column 1 of the patent, when the housing is placed in the hole opposite the boundary between adjacent sloping formations which react different to neutron bombardment, the response of each crystal detector will depend upon the nature of the formation which is opposite that detector, and from the simultaneous record which is made from the outputs of these detectors, the amount of inclination of the formation can be ascertained. The Scherbatskoy invention anticipates the combination of three or four detectors; each assigned to receive radiation readings from a specific radial portion of the well borehole wall. The patent also describes the use of gyroscopic orientation devices for determining the angular position of the instrumentation.
The problems associated with each of the above-referenced patents involve both the structure of the gamma ray detection devices and their ability to function in narrower cased boreholes and the use of chemical based radiation sources which have universal safety concerns that dramatically increase the cost of the operation. None of the above referenced patent disclosed devices are known to have been developed into workable systems. This is likely due to ineffective source outputs, and the random scatter of low and high energy levels of the neutrons, also hole riggosity affecting the shallow depth of investigation of the referenced devices.